Vehicle headlamps are commonly made with small, intense light sources. These light sources may be either tungsten halogen filament sources or high intensity discharge sources. Most of the generated light is controlled and directed by the reflector to be projected as a properly formed light beam. However, a portion of the light from the source goes directly forward and cannot be controlled by the reflector. Another portion of the light is reflected from nearby supports and wall structures that are closer to the source than is the reflector. These nearby objects then act as if they were secondary light sources acting as what is called parasitic sources. The directly projected light, and the reflections from parasitic sources are usually uncontrolled, and result in glare. It is frequently preferred to block this light with a light shield to limit the resulting glare.
The light and heat reflected from the center of the reflector normally adds to the heat and light coming directly from the light source, to heat in a center spot of the exterior lamp lens. The center of the lens can then suffer heat stress. Again, it is frequently preferred to shield this light to protect the exterior lens.
The light block or light shield may be a wall or similar structure placed intermediate the light source and the exterior. Light shields commonly have a cup shape. The light received in the light shield should not be reflected back out in an uncontrolled manner, so it is common to coat the inside surface of the light shield with a light absorbing material. The light shield frequently absorbs the received light and converts it to heat. As a result, the light shield becomes hot.
It has been found that over the life of a headlamp, the light absorbing material coating the light shield can either quickly or over time outgas material as the light shield bakes during lamp operation. The outgased material migrates in the enclosed headlamp, and condenses on the other structures, the reflector, the inside surface of the exterior lens, and even the light source itself. The resulting film may color the light or reduce the total amount of projected light. The headlamp then looks dingy, and performs less well. There is then a need for an inexpensive light shield coating that does not outgas during the life of operation.